There is a very popular open source command-line downloader called youtube-dl. which does exactly that. It grabs the actual video and audio file links from a given YouTube link – or any other popular web video site like Vimeo, Yahoo! Video, uStream, etc.
In order to find the video stream, you'd have to parse the HTML and JS code of the video page to find the relevant initialization code, and then from there try to find the link to the actual MP4 file. It might be there in
For HTML5 progressive download video, the actual source file is usually mentioned directly in the source child of the video tag, so if you'd search the page for mp4 or similar. For example on German news show Tagesschau 100. you'll find:
For more advanced playback technologies like MPEG DASH or Apple's HTTP Live Streaming (HLS), you have to parse a meta-information file to get the actual video stream. The meta file ( .mpd for example in DASH, and .m3u8 for HLS) will contain links to segments of video and audio, which you'd later have to combine to get a playable file.
There's no general solution for this. It requires careful inspection and debugging of the target site.
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